A plastic molding machine typically has two mold halves that must be selectively opened and closed during an operational cycle. After a preceding article has been formed and removed, one mold half must be moved rapidly toward its cooperative mate at relatively low force to initially close the mold. Thereafter, high force is required through a short stroke to clamp the mold in a closed position before plastic material is injected therein.
To meet these requirements with electric motor drives requires either an excessively large motor to provide the high force needed to close the mold, or a non-linear toggle mechanism to approximately match the performance of a smaller motor to the load requirements.
Recent improvements avoid these alternatives by effectively introducing a type of selectable “gear shift” in the mold-closing drive. This “gear shift” operates uni-directionally (i.e., only as the mold is being closed and clamped, but not as the mold is being unclamped an opened) through a direct-acting motor-driven ball-screw to rapidly advance one mold half toward the other, followed by a hydrostatic force amplifier or intensifier in which the motor-driven screw moves a small piston that hydraulically communicates with a large piston (which was been bypassed during the rapid mold movement) to develop the high force necessary to clamp the mold.
U.S. Pat. No. 4,968,239 (facially assigned to Fanuc Ltd.) discloses an electric motor drive for a plastic molding machine. This drive provides a type of “manual transmission” by providing different ratio gear trains which can be alternately selected by means of electromagnetic clutches. However, this reference does not teach the use of a hydrostatic transmission to selectively couple the large-stroke low-force gear train to the short-stroke high-force gear train.
U.S. Pat. No. 5,345,766 (facially assigned to Engel Maschinenbau GmbH) discloses a motor/ball-screw drive for a plastic molding machine in which the motor drives through a single-sided piston which is spring loaded to a fixed position on the machine platen. When the driving force requirements exceed the spring loading, the piston is moved relative to the platen to displace fluid into a parallel single-sided larger-area piston-and-cylinder arrangement to produce a hydrostatically-amplified clamping force. Alternative valving means are described to allow the filling of the large cylinder as the platen is advanced. However, this reference does not appear to teach or suggest that the operation may be reversed when it is desired to open the mold.
U.S. Pat. No. 6,439,875 B1 (facially assigned to Kabushiki Kaisya Meiki Seisakusyo) also discloses a motor-driven ball-screw arrangement in which the motor drives through a single-sided piston which is spring loaded to a fixed position. When the driving force requirements exceed the spring loading, the piston is moved relative to the platen to displace fluid into a parallel single-sided larger-area piston-and-cylinder arrangement to produce a hydrostatically-amplified clamping force. However, the small intensifier piston may be blocked by a solenoid valve, and the fluid supplied to the clamping piston is also controlled by a solenoid valve. Additionally, the large clamping piston has a short stroke that may be selectively coupled to the moving member at any position. Here again, this reference does not appear to teach or suggest that the operation may be reversed when it its desired to open the mold.
Recent improvements in injection molding processes include the technique of bringing the mold halves to an almost-closed position, injecting molten plastic, and then driving the mold closed to compress the plastic. This process requires that the mold actuator be moved precisely to a predetermined position and held there as plastic is injected, followed by advancing to the fully-closed position against the high compression load. The actuator must, therefore, be capable of closed-loop servo position control in both the low- and high-force modes. Closed-loop control, in turn, necessitates that the actuator be controllable in both directions.
Additionally, after an article has been molded by this method, the breakaway mold opening force that is needed to initially crack the mold may be substantially greater than the force for which the direct motor/screw drive would preferably be designed for normal rapid motion of the mold platen, thus requiring intensified force in the reverse direction to break open the mold. This is followed by a long-stroke low-force movement of one mold half relative to the other to fully open the mold.
Accordingly, it would be generally desirable to provide improved apparatus for moving one mold half relative to another, which apparatus affords the capability of a high-speed low-force approach as one mold half moves toward the other, including holding a predetermined position under closed-loop servo control, followed by a low-speed high-force clamping as the two mold halves are clamped together, and a low-speed high-force breakaway drive as the two mold halves are initially unclamped followed by a high-speed low-force movement of one mold half away from the other.